(1) Field of the Invention
The present invention relates to an improvement of synthetic resin insulators comprising a fiber-reinforced plastic rod or pipe (hereinafter, referred to as reinforced plastic rod), overcoats consisting of an elastic insulating material, and holding metal fittings.
(2) Description of the Prior Art
A reinforced plastic rod, reinforced with bundles of fibers or knitted fiber bundles in their axial direction, has a resistance against very high tensile stress and an extremely high strength-to-weight ratio. While, elastic insulating materials, such as silicone rubber, ethylene-propylene rubber, polyethylene, polypropylene, ethylene-propylene copolymer, cycloalipatic epoxy, acrylic, polyfluoroethylene and the like, occasionally mixed with an inorganic filler having a low decomposition temperature, such as alumina trihydrate or the like, have excellent weather resistance and tracking resistance, recently there have been made various investigations for producing light and high-strength synthetic resin insulators by combining these elastic insulating materials. As a typical synthetic resin insulator, there has been known an insulator comprising a reinforced plastic rod 1, a large number of superposed overcoats 3 made of an elastic insulating material and fitted to the rod 1, each overcoat 3 being provided at its outside with one shed, and grease 6 filled in the interface 4 between the reinforced plastic rod 1 and the overcoats 3, as illustrated in FIG. 1, FIG. 2a and FIG. 2b.
However, the conventional synthetic resin insulator, wherein a large number of individual overcoats 3 are superposed, is assembled in the following manner in order to prevent the leakage of grease 6 from the interface 4 or the penetration of water or the like into the interface 4. That is, overcoats 3 having an inner diameter smaller than the outer diameter of a reinforced plastic rod 1 are used in order to fasten tight always the reinforced plastic rod 1 and further the overcoats 3 are compressed in their axial direction between both holding metal fittings 2 and 2 to cause pressure between adjacent overcoats 3 and 3. As a result, the overcoats 3 are always elongated to the circumferential direction. Such elongated state promotes the breakage of molecular chain of elastic insulating material due to oxygen, ultraviolet ray and the like, and the electric insulating material in elongated state is apt to be easily deteriorated. Particularly, the shoulder x at the contact portion 5 of adjacent overcoats 3 is easily deteriorated by oxidation due to its large specific surface area as shown in FIG. 2a. Moreover, as the overcoats 3 are compressed in their axial direction, stress is concentrated into the shoulder x.sub. 1 and the shoulder x.sub.1 is elongated in a large amount and is apt to be deteriorated more easily. In general, this erosion proceeds in a direction perpendicular to the stretching direction. In addition, the shoulder x.sub.1 is eroded by the minute discharges due to leakage current, which flows on the overcoat surface during rainfall, as shown by the mark x.sub.2 in FIG. 2b, and the erosion grows rapidly in the form of a groove in a direction perpendicular to the stretching direction, that is, towards the interface 4 between the reinforced plastic rod 1 and the overcoats 3 in combination with the above-described deterioration of the shoulder. This directional erosion reaches the interface 4 between the overcoat 3 and the reinforced plastic rod 1 in a very short period of time to cause leakage of the grease 6 and penetration of water easily, and to promote insulation breakdown of the interface 4, and further to erode and break the reinforced plastic rod. As a result, the function of the insulator is lost. In this case, the deterioration speed of the function of the insulator due to erosion depends upon the erosion speed at the contact portion of adjacent overcoats 3.
Furthermore, when the insulator is practically used in the power transmission line, the insulator is exposed to the direct ray of the sun to cause temperature rise of the insulator, and grease 6 filled in the interface 4 is expanded due to the temperature rise to expand the overcoat 3. In this case, since airtightness between adjacent overcoats superposed one upon another is secured merely by the action of compression force in the axial direction of the overcoats, the expanded grease 6 leaks from the contact portion 5 of adjacent overcoats 3. Moreover, when a hot-line washing is carried out by the use of high-pressure water in order to wash away pollutant adhered to insulators used in a substation or the like in a region wherein insulators are violently polluted, the overcoats 3 are forcedly moved by the high-pressure water blown thereto to form gaps at the contact portion 5 of adjacent overcoats 3 and 3, and water is penetrated into the interface 4 through the gaps. As described above, there are many problems. In order to overcome these problems, there has been proposed an insulator, wherein a reinforced plastic rod 1 is bonded with overcoats 3 at the interface 4 with an adhesive and adjacent overcoats 3 are bonded with each other at the contact portion 5 with an adhesive. However, in this insulator, since the adhesive is generally an active material, the adhesive, even after solidified, is apt to be deteriorated more easily than the overcoat materials, and when the adhesive is exposed to the external atmosphere at the contrast portion 5 of adjacent overcoats, the adhesive layer is firstly deteriorated by the action of the above-described ultraviolet ray and oxygen and water in the external atmosphere, followed by the erosion due to minute discharges, to form gaps in the adhesive layer; and the shoulder x.sub.1 which has a large specific surface area and is liable to be oxidized and deteriorated, is successively eroded and deteriorated. This erosion reaches the interface 4 in a short period of time similarly to the above-described insulator, wherein grease 6 is filled in the interface 4, to cause insulation breakdown at the interface 4 and further to erode gradually the reinforced plastic rod 1, resulting in the separation of the insulator. Therefore, the insulator has serious drawbacks.
Further, there has been known an insulator produced by molding directly an individual overcoat 3 having one shed 8 on a reinforced plastic rod 1 by means of a mold 12 and repeating this molding to form the whole overcoats into substantially a unitary structure as illustrated in FIGS. 3a and 3b. However, in this insulator, the bonded plane 13 of adjacent overcoats 3 and 3 formed in every molding is weak chemically and mechanically and is apt to be oxidized and deteriorated, and moreover when the reinforced plastic rod 1 is elongated by the load applied to the insulator, the bond plane 13 of the overcoats 3 is often exfoliated, and therefore the insulator has serious drawbacks similarly to the above-described insulator. In order to solve the above-described drawbacks and problems, there has been proposed an insulator having a seamless unitary overcoat. However, a large size mold is required for producing the overcoat 3 corresponding to the increase of the length of insulator, and moreover it is very difficult to mold a long, slender, shed-shaped, peculiar overcoat, and mass production of the overcoat 3 having a length of more than 1 m is considered to be difficult.
Recently, the transmission voltage is raised more and more in order to obtain a high transmission efficiency, and an insulator having a long insulating distance has become necessary corresponding to the high transmission voltage.
Accordingly, when it is intended to obtain a desired insulating distance by using relatively short seamless unitary insulators, a large number of the insulators must be connected. There are many problems, namely, the insulating distance must be long in an amount corresponding to the lengths of respective holding metal fittings. Therefore, a tall steel tower which is required is expensive. Moreover, the weight of the insulator assembly increases corresponding to the increase of the number of holding portions, and further the respective holding metal fitting portions form weak points due to concentration of mechanical stress and electric stress, and hence the reliability of the insulator is lost when a large number of the weak points are formed.